A mechanical double-action press machine is normally provided with a regulating mechanism which regulates press load. This regulating mechanism, as shown in FIG. 1, comprises hydroblank holder 4, protective cylinder 8, a pneumatic device 12 and hydraulic device 15.
The hydroblank holder 4 incorporates piston 1 and has air chamber 2 and oil chamber 3 defined by the piston 1. The protective cylinder 8 defines oil chamber 7 at connection part between connection screw 5 and outer slide 6. The pneumatic device 12 includes pneumatic source 9, pressure regulating valve 10 and accumulator 11 and supplies a predetermined pneumatic pressure to oil chamber 2 in hydroblank holder 4. The hydraulic device 15 includes hydraulic source 13 and check valve 14 and supplies a hydraulic pressure to oil chamber 3 in hydroblank holder 4 and oil chamber 7 in protective cylinder 8.
When pneumatic pressure supplied to air chamber 2 in hydroblank holder 4, i.e., regulator pressure, is set to a high level in the mechanical double-action press machine equipped with such regulating mechanism, the hydraulic pressure does not operate and the incremental load produced by the pressure application of the press machine is transmitted directly to a press die through outer slide 6 to effect a mechanical zone forming operation. Here, the outer load is determined primarily by the die height.
On the other hand, if the regulator pressure supplied to air chamber 2 is adequately balanced against the hydraulic pressure, the above mentioned incremental load is absorbed by the operation of piston 1 of hydroblank holder 4. Therefore, the load is transmitted to the press die through hydraulic pressure to effect the hydraulic zone forming operation. Here, the outer load is determined by the die height and the regulator pressure.
The values of the die height or the regulator pressure are determined through the procedure of finding the optimal processing condition beforehand by a trial pressing. The values determined in the above procedure are set in the press machine in an actual production line and the press machine executes pressing in accordance with the set conditions.
However, there exist unavoidable mechanical differences between the two press machines. The regulatory pressure is set manually by an operator monitoring meters; hence, it is difficult to effect uniform regulation in practice.
Consequently, press-forming defects, such as surface distortion and cracking are produced by unstable dynamic characteristics of the outer load, varying from the ideal condition. Therefore, in the prior art, a time-consuming process of fitting and adjusting the press die has been necessary.